[jonrx]

AFAIK Hydro-Quebec already does something like this.

http://www.hydroquebec.com/learning/transport/grandes-distan...

Granted, there's not much details :

« The technology used to transmit direct current is not the most common. However, it can be advantageous for isolating alternating-current systems or controlling the quantity of electricity transmitted. Hydro-Québec has a direct-current line (which goes from the Baie-James region to Sandy Pond, near Boston) as well as many direct-current interconnections with neighboring systems. »